Objective of this research is the study and comparison of the antigens of axenically cultured Giardia trophozoites, isolated from man and from several lower animal species. Man and many other animals are parasitized by these intestinal protozoa, many of which cannot be distinguished from each other morphologically. The question as to whether or not other methods can help differentiate these organisms is of epidemiologic importance, inasmuch as we do not know whether man contracts this infection only from other humans with the disease, or whether lower animals, including dogs, cats and beavers, may also serve as a source of human infection. The work to date on this project has involved preparation of Giardia antigen from man, cat, and guinea pig in sufficient amounts to be used for the next step in these studies, namely the immunization of rabbits and the preparation of anti-Giardia antiserum. That antiserum in turn will be used in the comparative studies to follow.